The present invention relates to a method and apparatus for controlling the diameter of a silicon single crystal and more particularly to a silicon single crystal diameter controlling method and apparatus whereby in the continuous manufacture of a silicon single crystal while rotating it relative to a crucible, the diameter of the pulled single crystal is controlled during the pulling of the crystal.
Referring to FIG. 2 of the accompanying drawings, there is illustrated a schematic diagram showing a single crystal manufacturing apparatus according to the Czochralski method which is well known as a manufacturing method of single crystals of such semiconductor as silicon. It is to be noted that the Czochralski method is widely called as the CZ method and also it is generally referred to as a crystal pulling method.
In the Figure, during the manufacture of a silicon single crystal, contained in a crucible 2 is molten silicon 3 which is heated and melted by a heater 4 so that while rotating a silicon single crystal 1 in a direction opposite to the direction of rotation of the crucible 2 by a turning mechanism which is not shown, the silicon single crystal 1 is gradually pulled by a pulling apparatus 7 and the crystal is grown in the interfacial region between the molten silicon 3 and the silicon single crystal 1. This silicon single crystal 1 is grown from a seed crystal 1a by crystal growth and the seed crystal 1a is supported by a seed crystal holder 9 connected to the pulling apparatus 7. In the description to follow, this silicon single crystal under growth is referred to as a pulled single crystal. Also, the pulling apparatus 7 includes mechanisms for respectively moving vertically and rotating the silicon single crystal 1 and it is composed of a motor controller 8, a pull motor 9, etc. Thus, it is a well known apparatus and any detailed explanation of its construction will be omitted. The crucible 2 is supported by a lifting mechanism which is not shown in addition to the previously mentioned turning mechanism and it is designed so that even if a crystal is grown, for example, the liquid level is prevented from lowering but maintained constant by the lifting mechanism and the temperature distribution in the vicinity of the liquid surface is not varied.
Since the pulled single crystal 1 manufactured by the CZ method is processed into a cylindrical silicon single crystal ingot later on, it is required that the pulled single crystal 1 is pulled in such a manner that its diameter is substantially the same throughout the body portion on the whole. For this purpose, a diameter control is effected such that the diameter of the pulled single crystal 1 is directly measured during its pulling by such optical means as a video measure 5 and the pulled single crystal 1 is pulled while adjusting the pull rate in a manner that the diameter is maintained at the desired value. In this case, the optical means is attached to the crystal pulling apparatus so that the fusion ring appealing at the boundary of the pulled single crystal 1 and the liquid surface is measured obliquely from above with a single measuring line and the diameter of the pulled single crystal 1 is measuring directly.
The above-mentioned diameter control of a pulled single crystal through the adjustment of the pull rate is performed by comparing the actual measured diameter value from the optical means and the desired diameter value so that the resulting deviation is subjected to the well known PID control to calculate a pull rate as a manipulated variable and this manipulated variable is applied as a command to the motor controller 8, thereby controlling the pull rate.
With the above-mentioned silicon single crystal diameter control method and apparatus described above, it is general that if, for example, the pull rate of the crystal pulling apparatus is changed, the response of the silicon single crystal or the change of the diameter of the pulled single crystal is such that a considerable time is required before its effect is exerted (the dead time is long) and also the rate of change is small (the change or response speed is low). Thus, where the measured diameter value is compared with the desired diameter value to effect the PID control, a timely manipulation of the pull rate is required in order to accurately grasp a change in the diameter of the pulled single crystal and therefore an advanced control giving emphasis to the derivative term (derivative action) is required. However, considerable noise is included in the measured diameter value of the pulled single crystal measured by the optical means and there is the danger of the pull speed being varied excessively under the effect of such noise, Thus making it impossible to effect a control emphasizing the derivative term. Thus, there is a limit to the control of the crystal diameter variation and the yield of the crystal tends to deteriorate.
In addition, the adjustment of the PID parameters on the spot is essential and thus it is necessary to wastefully pull the silicon single crystal for the purpose of PID parameter adjustment alone.